I. Field of the Invention
The present invention relates to multiple access communication systems, such as wireless data or telephone systems, and satellite repeater type spread spectrum communication systems. More particularly, the invention relates to a method and apparatus for using multiple orthogonal codes to generate spread spectrum communication signals. The invention further relates to a method of using shift keying of multiple Walsh function code sequences for signal modulation in code division spread spectrum type communication systems to provide system users with improved energy metrics for non-coherent signal demodulation.
II. Description of the Related Art
A variety of multiple access communication systems have been developed for transferring information among a large number of system users. Techniques employed by such multiple access communication systems include time division multiple access (TDMA), frequency division multiple access (FDMA), and AM modulation schemes, such as amplitude companded single sideband (ASCII), the basics of which are well known in the art. However, spread spectrum modulation techniques, such as code division multiple access (CDMA) spread spectrum techniques, provide significant advantages over the other modulation schemes, especially when providing service for a large number of communication system users. The use of CDMA techniques in a multiple access communication system is disclosed in the teachings of U.S. Pat. No. 4,901,307, which issued Feb. 13, 1990 under the title "SPREAD SPECTRUM MULTIPLE ACCESS COMMUNICATION SYSTEM USING SATELLITE OR TERRESTRIAL REPEATERS," is assigned to the assignee of the present invention, and is incorporated herein by reference.
The U.S. Pat. No. 4,901,307 patent discloses a multiple access communication system technique in which a large number of generally mobile or remote system users each employ a transceiver to communicate with other system users or desired signal recipients, such as through a public telephone switching network. The transceivers communicate through satellite repeaters and gateways or terrestrial base stations (also sometimes referred to as cell-sites or cells) using code division multiple access (CDMA) spread spectrum type communication signals. Such systems allow the transfer of various types of data and voice communication signals between system users, and others connected to the communication system.
Communication systems using spread spectrum type signals and modulation techniques, such as disclosed in U.S. Pat. No. 4,901,307, provide increased system user capacity over other techniques because of the manner in which the full frequency spectrum is used concurrently among system users within a region, and `reused` many times across different regions serviced by the system. The use of CDMA results in a higher efficiency in utilizing a given frequency spectrum than achieved using other multiple access techniques. In addition, the use of wide band CDMA techniques permits such problems as multipath fading to be more readily overcome, especially for terrestrial repeaters.
Pseudonoise (PN) modulation techniques used in wide band CDMA signal processing provide a relatively high signal gain which allows spectrally similar communication channels or signals to be more quickly differentiated. This allows signals traversing different propagation paths to be readily distinguished, provided any path length difference causes relative propagation delays in excess of the PN chip duration, that is, the inverse of the bandwidth. If a PN chip rate of say approximately 1 MHz is used, the full spread spectrum processing gain, equal to the ratio of the spread bandwidth to system data rate, can be employed to discriminate between signal paths differing by more than one microsecond in path delay or time of arrival. This differential corresponds to a path length differential of approximately 1,000 feet. A typical urban environment provides differential path delays in excess of one microsecond, and some areas upwards of 10-20 microseconds in delay.
The ability to discriminate between multipath signals greatly reduces the severity of multipath fading, although it does not typically totally eliminate it because of occasional paths with delay differentials of less than a PN chip period. The existence of low delay paths is more especially true for satellite repeaters or directed communication links where multipath reflections from buildings and other terrestrial surfaces is greatly reduced. Therefore, it is desirable to provide some form of signal diversity as one approach to reducing the deleterious effects of fading and additional problems associated with relative user, or repeater, movement.
Generally, three types of diversity are produced or used in spread spectrum type communication systems, and they are time, frequency, and space diversity. Time diversity is obtainable using data repetition, time interleaving of data or signal components, and error coding. A form of frequency diversity is inherently provided by CDMA in which the signal energy is spread over a wide bandwidth. Therefore, frequency selective fading affects only a small part of the CDMA signal bandwidth.
Space or path diversity is obtained by providing multiple signal paths through simultaneous links with a mobile user through two or more base stations, for terrestrial-based repeater systems; or two or more satellite beams or individual satellites, for space-based repeater systems. That is, in the satellite communication environment or for indoor wireless communication systems, path diversity may be obtained by deliberately transmitting or receiving using multiple antennas. Furthermore, path diversity may be obtained by exploiting a natural multipath environment by allowing a signal arriving over different paths, each with a different propagation delay, to be received and processed separately for each path.
If two or more signal reception paths are available with sufficient delay differential, say greater than one microsecond, two or more receivers may be employed to separately receive these signals. Since these signals typically exhibit independent fading and other propagation characteristics, the signals can be separately processed by the receivers and the outputs combined with a diversity combiner to provide the final output information or data, and overcome problems otherwise existent in a single path. Therefore, a loss in performance only occurs when the signals arriving at both receivers experience fading or interference in the same manner and at the same time. In order to exploit the existence of multipath signals, it is necessary to utilize a waveform that permits path diversity combining operations to be performed.
Examples of using path diversity in multiple access communication systems are illustrated in U.S. Pat. No. 5,101,501 entitled "SOFT HANDOFF IN A CDMA CELLULAR TELEPHONE SYSTEM," issued Mar. 31, 1992, and U.S. Pat. No. 5,109,390 entitled "DIVERSITY RECEIVER IN A CDMA CELLULAR TELEPHONE SYSTEM," issued Apr. 28, 1992, both assigned to the assignee of the present invention, and incorporated herein by reference.
The CDMA techniques disclosed in U.S. Pat. No. 4,901,307 contemplate the use of coherent modulation and demodulation for both communication directions or links in user-satellite communications. In communication systems using this approach, a pilot carrier signal is used as a coherent phase reference for gateway- or satellite-to-user and base station-to-user links. The phase information obtained from tracking the pilot signal carrier is then used as a carrier phase reference for coherent demodulation of other system or user information signals. This technique allows many user signal carriers to share a common pilot signal as a phase reference, providing for a less costly and more efficient tracking mechanism. In satellite repeater systems, the return link generally does not require a pilot signal for phase reference for gateway receivers. In a terrestrial wireless or cellular environment, the severity of multipath fading and resulting phase disruption of the communication channel, generally precludes use of coherent demodulation techniques for the user-to-base station link, where a pilot signal is not typically used. However, the present invention allows the use of both noncoherent modulation and demodulation techniques as desired.
While terrestrial based repeaters and base stations have been predominantly employed, future systems will place more heavy emphasis on the use of satellite based repeaters for broader geographic coverage to reach a larger number of `remote` users and to achieve truly `global` communication service. Unfortunately, in the satellite environment, several factors sometimes have a negative impact on the usefulness of traditional signal diversity and frequency and phase tracking techniques.
Satellite repeaters operate in a severely power limited environment. That is, there is a reasonably limited amount of power that the satellite control and communication systems can practically have access to. This is based on factors such as satellite size, and energy storage mechanisms, among others. It is extremely desirable to reduce the amount of power required or being used by the communication system for anything other than actual data transfer for a system user or subscriber.
It is also possible that the system is servicing a relatively low number of actual users at any time, operating well below capacity. This circumstance could lead to a pilot signal that accounts for more than fifty percent of the power being used by the satellite portion of the communication system, resulting in a potentially unacceptable inefficiency in power use for satellite repeaters. In this latter situation, the pilot signal becomes too `expensive` to maintain, and pilot signal power could actually be decreased by system operators to compensate.
However, regardless of the reason for implementation, reducing power for pilot signals reduces the ability to initially acquire the pilot signal at high speed and provide for very accurate tracking of the pilot carrier phase. This is especially true in satellite systems where Doppler and other effects increase the difficulty in tracking the pilot carrier accurately, as compared to terrestrial based repeater systems. It can readily be seen that if the power is not high enough, or if Doppler and other effects are large enough factors, system users may not be able to reliably obtain a desired level of tracking for the pilot signal and must use a non-coherent demodulation scheme. That is, energy allocated to the pilot is insufficient to accurately estimate, to some specified level, the phase of the signals for coherent demodulation, or maintain tracking. At the same time, pilot energy received at the Earth's surface may be low adjacent to the edges of some satellite beam spots due to antenna signal shaping and such.
Therefore, it is desirable to provide a method of acquiring or demodulating a spread spectrum communications signal using non-coherent demodulation techniques. It is desirable for such techniques to operate effectively for system users or subscribers in the presence of decreased pilot signal energy. This should apply even when the pilot energy has decreased to such a low energy level, either by design or because of propagation effects, as to be non-detectable for practical purposes. At the same time, this technique should not interfere with the effective use of pilot signal information when it is available, and should be highly compatible with other pilot signal and CDMA communication system protocols.